1. Field of the Invention
The invention relates to an oxidation resistant ceramic comprising carbon, silicon, oxygen and boron. More particularly, the invention relates to a lightweight, monolithic ceramic composition of carbon, silicon, oxygen and boron which is able to retain its shape and strength when exposed to an oxidizing environment at 1200.degree. C. and to a sol-gel method for its preparation comprising coating a carbon substrate with a sol comprising a mixture of di- and tetrafunctional siloxanes and a boron alkoxide, gelling the sol and heating the coated carbon substrate in an inert atmosphere to form the ceramic.
2. Background of the Disclosure
Reusable space vehicles, such as the space shuttle which must leave and reenter the earth's atmosphere, require exterior thermal insulation protection. The successful operation of the space shuttle required the development of light weight and very thermally efficient exterior insulation which had to withstand a wide variety of environments. During reentry back into earth's atmosphere, the insulation must maintain the vehicle's exterior structure below 175.degree. C. while experiencing substantial aeroconvective thermal environments which can heat the surface of the insulation to temperatures in excess of 1,000.degree. C. In space the thermal protection must insulate the vehicle from the deep and constant cold (e.g., -70.degree. C.) experienced while in orbit. In addition to thermal and aeroconvective environments, the insulation must also be able to withstand the mechanical stress associated with launch vibrations, acoustics, structural movement of the surface of the vehicle and of the ceramic insulation, and the landing impact.
Both rigid and flexible ceramic compositions have been used as thermal insulation for protecting space vehicles has included both rigid and flexible ceramic compositions, with a rigid carbon/carbon composite tile used on the leading edges of the vehicle. Most of the ceramic composites are very porous in order to keep the weight down to a reasonable level and some have a void volume of over 90%. More advanced ceramic insulation has been and continues to be developed for space shuttles and other reentry space vehicles in which either all or a substantial portion of the insulation comprises one or more refractory metal oxides, carbides, borides, silicides, borosilicates and nitrides. Ceramic materials currently in use or in development include high purity silicon dioxide, aluminum oxide, silicon carbide, aluminosilicate, aluminoborosilicate and zirconium diboride as illustrative, but nonlimiting examples.
However, a reinforced, rigid carbon-carbon ceramic composite continues to be used on the leading edges of space vehicles as disclosed, for example, in an article by Dotts, et. al. "The Toughest Job Ever", p.616-626 in Chemtech, October, 1984. The reinforced carbon-carbon (RCC) structure disclosed in this article begins with a rayon cloth which is graphitized, impregnated with a phenolic resin and pyrolyzed, then impregnated with furfural alcohol and pyrolyzed again, with this process repeated until a density of 90 to 100 lb/ft.sup.3 is achieved. The RCC is protected from oxidation by converting the outer surface to silicon carbide in a diffusion coating process. Additional oxidation resistance is provided by impregnation with tetraethylorthosilicate cured to form a silica residue to further reduce the area of exposed carbon, followed by sealing with a sodium silicate-SiC mixture to fill any remaining surface voids or cracks. This process is time consuming, costly, results in a composite which is much too heavy for today's more advanced payload and distance requirements and still results in a RCC material which is susceptible to oxidation where the carbon substrate is exposed. Hence, it would be an improvement to the art if a lightweight, oxidation resistant material could be made which possesses the light weight and high temperature thermal stability and insulating advantages of RCC, without its disadvantages.